1. Field of the Invention
This invention relates to an optical network.
2. Related Art
Currently, in the United Kingdom, the telecommunications network includes a trunk network which is substantially completely constituted by optical fibre, and a local access network which is substantially completely constituted by copper pairs. In future, it would be highly desirable to have a fixed, resilient, transparent telecommunications infrastructure all the way to customer premises, with capacity for all foreseeable service requirements--or at least to points (e.g. the curb) closer to such customer premises. One way of achieving this would be to create a fully-managed fibre network for the access topography. An attractive option for this is an optical tree access network, such as passive optical networks (PONs) which incorporate single mode optical fibre and no bandwidth-limiting active electronics.
In a PON, a single fibre is fed out from a head-end (exchange), and is fanned out via passive optical splitters at cabinets and distribution points (DPs) to feed optical network units (ONUs). The ONUs can be in customers' premises, or in the street serving a number of customers. The use of optical splitters enables sharing of the feeder fibre and the exchange-based optical line termination (OLT) equipment, thereby giving PONs cost advantages. At present, simplex deployment of PONs is the preferred option, that is to say separate upstream and downstream PONs are provided whereby each customer has two fibres. Although simplex working increases the complexity of the infrastructure due to the two fibres per circuit required, it benefits from a low optical insertion loss (due to the absence of duplexing couplers), and a low return loss, since such systems are insensitive to reflections of less than 25 dBm with separate transmit and receive paths. However, duplex PONs where one single fibre carries traffic in both directions are also possible. Typically, a PON has a four-way split followed by an eight-way split, so that a single head-end fibre can serve up to 32 customers.
In a known arrangement--TPON (telephony over a passive optical network)--a head-end station broadcasts time division frames to all the terminations on the network. The transmitted frames include both traffic data and control data. Each termination recognises and responds to appropriately-addressed portions of the data in the broadcast frames, and ignores the remainder of the frames. In the upstream direction, transmission is by time division multiple access (TDMA) where each termination transmits data in a predetermined timeslot, so that the data from the different terminations are assembled into a TDMA frame of predetermined format.
The present applicant has developed a bit transport system (BTS) for use in a PON which operates using TDMA. The BTS is described in our European patent specifications 318331, 318332, 318333 and 318335.
One feature necessary to such a network is the provision of compensation for the differing delays and attenuations associated with the different distances of the various terminations from the head-end station. To this end, each termination is arranged to transmit a ranging pulse timed to arrive in a respective predetermined portion of the upstream TDMA frame. The head-end station is arranged to monitor the timing, i.e. the-phase and the amplitude of the arrival of the pulse from each of the terminations, and to return servo-control signals to the terminations to retard or advance their transmissions as appropriate, and to adjust their launch power. This ranging and levelling process is particularly important during set-up of a PON system, or when a PON system is upgraded, or when a PON system is returned to use after a fault has been repaired. In such cases, the ranging and levelling process takes a finite time (the round trip delay) which is dependent upon the distance from the head-end station to the terminations. This round trip delay from the terminations to the head-end station and back to the terminations to effect ranging and levelling is known as the dead zone. This is because the dead zone represents the time during which PON customers can get no service as the PON is being used exclusively for ranging and levelling. For a simple PON of the type described above, in which a head-end station is connected to up to 32 terminations over a distance of typically 6-8 km, the dead zone is only 60-80 ms, and this does not represent a major problem.
Recently, however, the PON principle has been expanded to form what is known as the SuperPON concept, in which high power optical amplifiers are used to allow very large, high split PONs to be built. For example, the use of optical amplifiers (such as fibre amplifiers) permits up to 3500 customers to be connected to a single head-end station over distances of up to 200 km. In this case, the dead zone is of the order of 1 ms to 2 ms, and this does give rise to significant loss of service to customers of such a SuperPON.
Unfortunately, optical amplifiers can only be used on a downstream SuperPON, as the use of amplifiers on an upstream SuperPON would cause noise problems resulting from the superposition of amplified stimulated emissions (ASEs) from the amplifiers. One way of providing amplification in an upstream SuperPON is to replace the last level of split (that is to say the level of split nearest the headend) by a repeater. This device converts incoming optical signals to electrical signals, amplifies them, and converts the amplified electrical signals to optical signals for onward transmission. Note that such networks are often loosely referred to as PONs, even though they may include electronic amplification and are not, therefore, strictly speaking, "passive".